Just to complete for Windows Vista or later here it is the command ready to use: forfiles /m *.png /c "CMD /c composite -compose Dst_Over -tile pattern:GRAY0 @FILE @FNAME.jpg"

I don't have Photoshop and I tried your solution first, thank you it works perfectly. If you want to add that for people working with XP that usually don't have the ForFiles utility, they can download it here. This is the batch ready to use: forfiles -m*.png -c"CMD /c composite -compose Dst_Over -tile pattern:GRAY0 @FILE @FNAME_WITHOUT_EXT.jpg"

Your solution is almost perfect for my case, the only thing is that if I have a long text inside .mainContent, it goes under the sidebar columns. I am trying to find a fix.